If you were super-super motivated, you could change all the Monster configs to match the Attack... configs. And then add in whatever mods Attack has that Monster does not (also adjusting their configs as necessary). But at the end of the day, all you would end up with is the Attack pack with some extra mods added in, and missing any world gen added by Monster.
It would be a painful experience, at best. Could be done, in theory, but I wouldn't want to take on the task. I'd suggest that you are far, far better off starting anew.